Recently, various services based on wireless communication networks have been provided owing to rapid progress in the areas of communication systems, computer networks and semiconductor devices. Also, users need high-level services as days go by, and there is a trend of explosive growth in wireless internet service markets all over the world. Accordingly, mobile communication systems in wireless communication networks have been developed to provide multi-media services, capable of transmitting various data, as well as voice services.
Now, wireless data services are commercialized based on technologies using code division multiple access (CDMA) 2000, evolution data only (EV-DO), wideband CDMA (WCDMA) and wireless local area network (WLAN). Recently, mobile phones are widely used in every home, and demands for wireless data are increasing constantly. To meet the demands, a method of providing mobile telecommunication with a small base station established indoors is introduced. With the small indoor base station, it is possible to access to a core network of mobile telecommunications networks through an indoor broad band network. The small mobile telecommunication base station is known as an indoor base station or femto-base station.
By introducing the indoor base station to a conventional telecommunication network including an outdoor base station, there have been developments on hybrid telecommunication networks including outdoor and indoor base stations. As a typical example, there is a long term evolution (LTE) network of which standard has been settled with the 3rd Generation Partnership Project (3GPP). Networks in accordance with the LTE standards (hereinafter, “LTE network”), have 100 Mbps and 50 Mbps data transmission rates at a down-link and an up-link, respectively. With respect to a delay, a user's data bandwidth is within 5 ms. Also, the LTE network has a low transmission delay and an improved transmission rate at cell boundaries.
A conventional LTE network includes an outdoor base station (Macro-eNB (enhanced Node B)), an indoor base station (Home-eNB), a mobility management entity (MME) and a self organizing & optimizing network (SON) server. Since the indoor base station and the outdoor base station access the core network independently, a user equipment (UE) can communicate through either the indoor base station or the outdoor base station. The MME is used for managing a handover and processing a call of the user equipment. The SON server performs establishment and optimizations of the outdoor/indoor base stations. Also, the SON server is an arbitrary server which provides basic parameters or required data to the respective base stations. Any functions of the MME and SON server can be performed with one network management device.
In the LTE network having the structure mentioned above, access to the indoor base station cell (femtocell) is granted to limited user equipments (subscribers). To put it concretely, the indoor base station can broadcast System Infoiniation Block type 1 (SIB 1), i.e., information on a femtocell which is managed by the indoor base station itself. The SIB 1 includes a closed subscriber group (CSG) indicator which denotes whether access to the femtocell is allowed or not. If the CSG indicator in the SIB 1 broadcasted by the indoor base station denotes a ‘True’ value, the communication is performed in a mode of the CSG, i.e., only specific subscribers are permitted to access the femtocell. If the CSG indicator denotes a ‘False’ value, the communication can be performed in another mode the opened subscriber group (OSG), i.e. all subscribers can access to the femtocell. If the CSG indicator denotes a true value, the user equipment can access the femtocell only when it is confirmed that the corresponding femtocell is included in a white list which shows femtocells accessible to the user equipment.
Like this, according to the CSG mode, only specific user equipment is permitted to access the femtocell and other user equipment is not permitted in any circumstance. Moreover, in the CSG mode, the access to the femtocell is never permitted to other user equipment but the specific user, even though the other user equipment has some imbalance between up-link (UL) path loss and down-link (DL) path loss. In this case, the user equipment can access only to the cell of the outdoor base station (macro-cell) instead of accessing an adjacent femtocell. Thus, lots of power should be consumed for an up-link transmission with the outdoor base station (macro-base station). Further, the up-link transmission between the user equipment and the outdoor base station causes considerable interference to the up-link transmission of the indoor base station.